Friday, January 22, 2010

High and Low Shot Games

I looked at low shot games in one of my Puck Prospectus articles from earlier this year, and I wanted to look at those games in a bit more detail. I took a sample of this season's games with the most and the least shots taken by one of the teams. For the most shots, I took the 40 highest totals, while for the least shots I took the top 38 (to avoid ties that would have extended the sample well past 40).

These numbers really show how a team's shooting percentage (or, conversely, the opposing goalie's save percentage) impacts the number of shots in the game. A high save percentage tends to cause higher shots against, while a lower save percentage tends to cause lower shots against.

I did not remove empty net goals. If I had done so, I'd guess that both the high and low shot teams would have had third period numbers that were more similar to their second period results.

The low shot teams shot the lights out in the first period, and then probably scored at about the the league average rate for the next two periods (excluding likely empty-netters). This indicates fairly strongly that the direction of causation runs from the percentages to the totals, rather than the other way around. The high shot teams scored at a below-average rate throughout the game, suggesting that very high shots against numbers do tend to go hand in hand with a high save percentage (although it could still be that an outstanding goalie performance is usually required for a team to take that many shots).

If the game score had a strong effect on the numbers, then that should show up in a breakdown of shots against by period:

The third period is when we would expect to see the strongest playing to the score effects. The third period is when the least shots were taken in low shot games. This again supports the theory that low shot games are usually a result of the scoreline and the percentages early in the game.

In contrast teams with high shot totals did not show any playing to the score effect, at least on the offensive side.

I also thought to look at the shots broken down by game situation, to see whether power play or shorthanded situations might have had a significant impact.

The low shot teams had a higher shooting percentage at all four game situations. In my view this supports the theory that shooting percentage has a strong impact on shots against. I suspect there may be some shot quality effect at even strength related to shots against. For example, the team that is taking more shots will likely be also taking more from the less dangerous scoring areas, and more of their shots will come from third/fourth liners and defencemen. However, it seems unlikely to me that a goalie would perform worse while shorthanded or on the power play simply because of a low overall number of shots against in the game.

I had a theory that one of the reasons that goalies often put up low save percentages when not facing very many shots was that they faced a higher percentage of power play shots. It turns out that the exact opposite was true, at least in 2009-10. One of the reasons that those teams took few shots in the first place was that they did not tend to have very many power play opportunities. The high shot teams had a situational distribution that is very similar to the overall average.

It seems apparent that save percentage has an impact on shots against. Teams that posted high shot totals tended to have very low shooting percentages early in the game, while the opposite is true for teams with low shot totals. There does appear to be some relationship between high shots against and a higher save percentage, although it is still somewhat uncertain how much that has to do with a potential shot quality effect and how much the shots against are caused by the high save percentage.

A very high shots against total tends to come in games where one team is significantly better than the other and where the better team has some incentive to keep playing for 60 minutes (i.e. the score is close or the team being outshot has the lead). That is usually only possible when the goalie on the weaker team is having a good day. After all, if the better team gets 3 or 4 goals and builds a comfortable lead then they usually shut it down somewhat and don't end up hitting the 40+ shot range.

It may be possible that goalies on weak teams get more chances to pad their stats with 40+ shots against games where a lot of the shots are of relatively low quality. It is possible that this counterbalances the likelihood that bad teams would tend to give up higher numbers of scoring chances against. I think more analysis needs to be done to determine whether there is an overall effect. All studies I've seen that look at seasonal averages for all teams show no relationship between save percentage and shots against, so I don't doubt that for most teams it all comes out even in the wash, but it remains to be seen whether there are unusual effects on outlying teams such as the Panthers or the Devils.

These numbers really show how a team's shooting percentage (or, conversely, the opposing goalie's save percentage) impacts the number of shots in the game. A high save percentage tends to cause higher shots against, while a lower save percentage tends to cause lower shots against."

So the save percentages for the opposing teams goaltenders would be...

I think that is more telling and yes it does show a correlation. I'm not convinced until it's done on a broader sample of teams and shot totals over a number of years. Also, I would like to see the shots per period.

The third period is when we would expect to see the strongest playing to the score effects. The third period is when the least shots were taken in low shot games. This again supports the theory that low shot games are usually a result of the scoreline and the percentages early in the game.

In contrast teams with high shot totals did not show any playing to the score effect, at least on the offensive side."

Good stuff I wonder why the second period is the magic period for low shot games. Again, I'd like to see shots per game. The save % is also very high in the second period for low shot games. Showing a correlation between percentage of shots and save %.

"The low shot teams had a higher shooting percentage at all four game situations. In my view this supports the theory that shooting percentage has a strong impact on shots against. I suspect there may be some shot quality effect at even strength related to shots against. For example, the team that is taking more shots will likely be also taking more from the less dangerous scoring areas, and more of their shots will come from third/fourth liners and defencemen. However, it seems unlikely to me that a goalie would perform worse while shorthanded or on the power play simply because of a low overall number of shots against in the game."

I think this just has to do with a goal is more of a detriment to a goaltender who faces fewer shots. The shorthanded numbers support this.

"I had a theory that one of the reasons that goalies often put up low save percentages when not facing very many shots was that they faced a higher percentage of power play shots. It turns out that the exact opposite was true, at least in 2009-10. One of the reasons that those teams took few shots in the first place was that they did not tend to have very many power play opportunities. The high shot teams had a situational distribution that is very similar to the overall average."

"It seems apparent that save percentage has an impact on shots against. Teams that posted high shot totals tended to have very low shooting percentages early in the game, while the opposite is true for teams with low shot totals. There does appear to be some relationship between high shots against and a higher save percentage, although it is still somewhat uncertain how much that has to do with a potential shot quality effect and how much the shots against are caused by the high save percentage."

Yes it seems apparent with this data but I just don't agree with that blanket statement. There needs to be a wider study and more information (teams, shots for, shots against, PPO, PPOA, Wins/losses).

"A very high shots against total tends to come in games where one team is significantly better than the other and where the better team has some incentive to keep playing for 60 minutes (i.e. the score is close or the team being outshot has the lead)."

Agreed.

"That is usually only possible when the goalie on the weaker team is having a good day. After all, if the better team gets 3 or 4 goals and builds a comfortable lead then they usually shut it down somewhat and don't end up hitting the 40+ shot range."

Usually, possible, somewhat. Not convinced all inferences based on limited data.

"It may be possible that goalies on weak teams get more chances to pad their stats with 40+ shots against games where a lot of the shots are of relatively low quality. It is possible that this counterbalances the likelihood that bad teams would tend to give up higher numbers of scoring chances against. I think more analysis needs to be done to determine whether there is an overall effect. All studies I've seen that look at seasonal averages for all teams show no relationship between save percentage and shots against, so I don't doubt that for most teams it all comes out even in the wash, but it remains to be seen whether there are unusual effects on outlying teams such as the Panthers or the Devils. "

I agree with this but I feel the studies should be conducted differently. Based on my studies a goalies save % is influenced by the percentage of games he faces in certain categories (below 20, 20-24, 25-29) and so on. Of the 5 goaltenders I've done research for (Hasek, Brodeur, Luongo, Belfour, Roy). Every goaltender (except Hasek who has a much higher 20-24 SPG on average and a lower 25-29 SPG) follows a save % similar to this-

What team/teams? Don't you think their coaching and style would be a huge influence? Why not all teams?

Sorry, that was just a poor turn of phrase, I meant to say the 40 highest shot totals recorded this season, i.e. by any team.

The save % is also very high in the second period for low shot games. Showing a correlation between percentage of shots and save %.

Not really, the league average is .911 this season, so .913 on that number of shots does not show a significant difference from league average.

Then when you break the percentage of games they faced in each season and apply it to these save %'s you get around a .03 differiential (on average) of expected to actual save %.

Yeah, but the question is whether the percentage of games they faced in each shot bracket is because of their teammates or because of their own play. The fact that the relationship does not seem to hold in aggregate over a season seems to suggest that the save percentage itself has a major effect.

About Me

I'm Philip Myrland, goalie stats blogger since 2007 and former writer for Hockey Prospectus. I call myself a contrarian because I rely mainly on stats-based analysis, think large sample sizes are more important than "big saves" and prefer to rank goalies based on save percentage rather than team success. In other words, pretty much the mainstream views of the hockey analytics community, but I've grown attached to the handle and plan to keep it as long as broadcasters everywhere keep bringing up GAA and wins. If you have any questions or comments contact me by email (contrariangoaltender@gmail.com). You can also follow me on Twitter (@tcghockey).